DC-DC power converters are utilized in situations where one DC voltage is converted to another DC voltage. In one application, that associated with PC based systems, the processor requires a fairly low voltage and a fairly high current. Rather than convert an incoming AC voltage down to a very low DC voltage and then route the low DC voltage across a PC board, a higher DC voltage is output by the power supply, routed around to the various components on the PC board and then, proximate to the processor, the voltage is down converted to a very low level on the order of 1.0 V. This requires a conversion device to be disposed proximate to one or more high current integrated circuits on the board.
Typical DC-DC converters are fabricated using a switching supply that utilizes a switched inductor or capacitor configuration with the input DC voltage switched to the input thereof with a periodically waveform operating at a preset switching frequency with a varying duty cycle. By sensing the output voltage and comparing it with a desired voltage, the duty cycle of the waveform can be adjusted to control the amount of current supplied to the reactive components. This control is facilitated with a negative feedback control loop.
There are two types of feedback loops, an analog feedback loop and a digital feedback loop. The analog feedback loop is well understood and provides some advantages over the other type of feedback loop, the digital feedback loop. Each of the feedback loops has associated therewith a voltage sense input for sensing the supply output voltage and a pulse width modulator (PWM) for generating switching pulses for driving switches. The sensed voltage is compared in the analog domain to a desired operating DC voltage to generate an error voltage that is reduced to essentially zero volts at regulation. To compensate for loop phase shift, there is provided a compensator. This provides some phase lead in the feedback loop for the purpose of loop stability. The digital controller portion of the digital feedback loop is similar to the analog feedback loop. The voltage signal sense input utilizes an analog-to-digital converter (ADC) to convert the output voltage to a digital value and then compare this to a desired voltage to determine the difference voltage as an error voltage. A digital compensator then provides some phase lead to the feedback to maintain stability in the control loop. This digital error voltage is then converted into a varying pulse width for output to the driving switches on the switching converter. This in effect is a digital-to-analog converter. Typical switching converters such as buck converters can utilize single or multiple phases to facilitate the switching operation.
During soft start the input voltage of the power supply is controlled from “0” to the reset point of the power supply. With analog controllers a capacitor is used to control the slope of a line between the “0” point and the reset point. With existing analog controllers only the slope of the line may be charged. It would be desirable to have some manner for more particularly controlling the input voltage during a soft start over that presently provided by analog controllers.